表面处理对蓝宝石衬底的影响

蓝宝石衬底是目前最为普遍的一种衬底材料,是生长GaN、ZnO材料最常用的衬底.本文用光学显微镜、原子力显微镜(AFM)和高分辨X射线双晶洐射对蓝宝石衬底进行了分析测试,系统研究了经过机械抛光、化学机械抛光、化学腐蚀等表面处理对蓝宝石衬底表面性能的影响.结果表明经过化学机械抛光随后再经腐蚀后的蓝宝石衬底的表面性能最好.     

Supersmooth and modified surface of sapphire crystals: Formation,characterization, and applications in nanotechnologies

The results of studying the state of the surface of sapphire crystals by a complex of methods in different stages of crystal treatment are considered by an example of preparing sapphire substrates with a supersmooth surface. The possibility of purposefully forming regular micro- and nanoreliefs and thin transition layers using thermal and thermochemical impacts are considered. The advantages of sapphire substrates with a modified surface for forming heteroepitaxial CdTe and ZnO semiconductor films and ordered ensembles of gold nanoparticles are described. The results of the experiments on the application of crystalline sapphire as a material for X-ray optical elements are reported. These elements include total external reflection mirrors and substrates for multilayer mirrors, output windows for synchrotron radiation, and monochromators working in the reflection geometry in X-ray spectrometers. In the latter case, the problems of the defect structure of bulk crystals sapphire and the choice of a method for growing sapphire crystals of the highest structural quality are considered.     

c面白宝石衬底表面形貌对气相传输平衡法制备的γ-LiAlO2层质量的影响

采用气相传输平衡(VTE)技术,在(0001)面白宝石衬底表面上成功地制备出单相γ-LiAlO2层.研究了白宝石衬底表面形貌对γ-LiAlO2层质量的影响,发现白宝石衬底的表面粗糙度和退火处理是两个影响γ-LiAlO2层质量的重要因素.要制备高质量的γ-LiAlO2层,适度的表面粗糙度是恰当的.对白宝石衬底进行退火处理,γ-LiAlO2层的择优取向变差.并对其中可能的机理进行了探讨.     

Influence of ensembles of gold nanoparticles on the growth of ZnO on the sapphire (0001) surface

Gold is deposited on sapphire (0001) substrates by vacuum sputtering to form an ensemble of nanoparticles on the surface. Then the substrates are transferred to a magnetron sputtering chamber to be heated to 650°C and coated by a ZnO layer. The resulting layers are investigated by scanning electron microscopy, atomic force microscopy, electron diffraction, and photoluminescence. It is established that the Au sublayer ensures a higher quality of the ZnO structure and enhances photoluminescence. The differences in the morphology of ZnO structures is explained by the Au aggregation during substrate heating before ZnO deposition.     

Specific features of the growth of A II B VI films on (0001)Al2O3 substrates

The structure and orientation of CdTe and ZnO films on sapphire have been investigated for different techniques of pregrowth substrate treatment. Polycrystalline CdTe films are found to grow of substrates unannealed or annealed in vacuum at a residual pressure P < 0.13 Pa. Epitaxial CdTe films with the sphalerite cubic structure, oriented parallel to the substrate by the (111) plane, grow on substrates annealed in air at a temperature of 1000°C or more and having a system of smooth terraces and steps on the surface. For ZnO films with a wurtzite hexagonal structure obtained by magnetron sputtering, a similar correlation between the structural quality and the regime of treatment of sapphire substrates is observed. It is shown that thermal annealing of (0001) sapphire plates in air is the optimal way of substrate preparation for growing epitaxial ZnO films with the base orientation. The obtained epitaxial CdTe films contain a certain amount of structural defects (mosaicity and twins), while the epitaixal ZnO films treated in the same way are close to perfect.     

Sapphire substrates with a regular surface relief

The lithography-free technique is proposed to obtain a regular relief in the form of a regular 2D system of 25-nm protrusions on a sapphire plate surface. The use of grid masks allows one to form a regular relief on the sapphire substrates of arbitrary area. The structure of crystal films formed by the sputtering of metal aluminum onto sapphire substrates with subsequent oxidation and annealing is thoroughly investigated and compared with the nanostructured (0001) sapphire wafer surface in the form of regular steps up to 5 nm in height with atomically smooth terraces.     

Electron microscopy studies of epitaxial MgB2 superconducting thin films grown by in situ reactive evaporation

The morphology and chemistry of epitaxial MgB₂ thin films grown using reactive Mg evaporation on different substrates have been characterized by transmission electron microscopy methods. For polycrystalline alumina and sapphire substrates with different surface planes, an MgO transition layer was found at the interface region. No such layer was present for films grown on MgO and 4-H SiC substrates, and none of the MgB₂ films had any detectable oxygen incorporation nor MgO inclusions. High-resolution electron microscopy revealed that the growth orientation of the MgB₂ thin films was closely related to the substrate orientation and the nature of the intermediary layer. Electrical measurements showed that very low resistivities (several μΩ cm at 300 K) and high superconducting transition temperatures (38 to 40 K) could be achieved. The correlation of electrical properties with film microstructure is briefly discussed.     

Epitaxial growth of ferroelectric PLZT [(Pb,La)(Zr,Ti)O3] thin films

Thin films of PLZT were epitaxially grown at around 700°C on sapphire and SrTiO₃ substrates: (111) PLZT  (0001) sapphire and (100) PLZT  (100) SrTiO₃. PLZT films on semiconductor substrates were also grown at around 620°C. The crystal quality of these PLZT films was investigated by X-ray diffraction, reflection electron diffraction, scanning electron microscopy (SEM), and Auger electron spectroscopy (AES). The epitaxy of PLZT films grown on different single crystal substrates is discussed. The refractive index of the film on the sapphire substrate was determined as 2.49₇ by an optical waveguide technique.     

Investigation of void formation beneath thin AlN layers by decomposition of sapphire substrates for self-separation of thick AlN layers grown by HVPE

Void formation at the interface between thick AlN layers and (0 0 0 1) sapphire substrates was investigated to form a predefined separation point of the thick AlN layers for the preparation of freestanding AlN substrates by hydride vapor phase epitaxy (HVPE). By heating 50–200 nm thick intermediate AlN layers above 1400 °C in a gas flow containing H₂ and NH₃, voids were formed beneath the AlN layers by the decomposition reaction of sapphire with hydrogen diffusing to the interface. The volume of the sapphire decomposed at the interface increased as the temperature and time of the heat treatment was increased and as the thickness of the AlN layer decreased. Thick AlN layers subsequently grown at 1450 °C after the formation of voids beneath the intermediate AlN layer with a thickness of 100 nm or above self-separated from the sapphire substrates during post-growth cooling with the aid of voids. The 79 μm thick freestanding AlN substrate obtained using a 200 nm thick intermediate AlN layer had a flat surface with no pits, high optical transparency at wavelengths above 208.1 nm, and a dislocation density of 1.5×10⁸ cm⁻².     

Homoepitaxial growth and electrical characterization of GaN-based Schottky and light-emitting diodes

Homoepitaxial growth of GaN epilayers on free-standing hydride vapor phase epitaxy (HVPE) GaN substrates offered a better control over surface morphology, defect density, and doping concentration compared to conventional heteroepitaxial growth. The FWHM of the (0 0 0 2) X-ray diffraction (XRD) rocking curve from homoepitaxial GaN was measured to be as low as 79 arcsec, much smaller than 230 arcsec for GaN grown on sapphire. Schottky diodes grown on GaN substrates exhibited sharper breakdown characteristics and much lower reverse leakage than diodes on sapphire. However, the homoepitaxial devices had poor scalability due to the presence of yield-killing defects originating from the substrate surface. Vertical InGaN/GaN light-emitting diodes (LEDs) on GaN substrates showed reduced series resistance and reverse leakage compared to lateral LEDs on sapphire. Wafer mapping demonstrated that the distribution of leaky homoepitaxial devices correlated well with that of macroscopic defects in the GaN substrates.     

The influence of strain and mosaic structure on electrical and optical properties of GaN films on sapphire substrates

Photoluminescence, electron transport measurements, and seven-crystal X-ray diffractometry have been used to analyze high mobility (intentionally silicon-doped) and highly resistive (unintentionally doped) GaN films grown on a-plane (11 0) sapphire substrates. Values of ω/2θ X-ray rocking curve FWHMs were seen which were lower than typically reported for GaN films on sapphire, and this has been attributed to the increased thickness of the analyzed films. Broadening was observed in the tail of the ω/2θ X-ray rocking curve of a Si-doped GaN film relative to that of unintentionally doped films, indicating diffuse scatter of the X-ray beam. Mosaic structure and curvature induced in the sapphire substrate by the subsequently grown GaN film was also observed.     

MOVPE homoepitaxy of high-quality GaN: Crystal growth and devices

Epitaxial growth on GaN bulk single crystal substrates sets new standards in GaN material quality. The outstanding properties provide insights into fundamental material parameters (e.g. lattice constants, exciton binding energies, etc.) with a precision not obtainable from heteroepitaxial growth on sapphire or SiC. With metalorganic vapor phase epitaxy (MOVPE) we realized unstrained GaN layers with dislocation densities about six orders of magnitude lower than in heteroepitaxy. By the use of dry etching techniques for surface preparation, an important improvement of crystal quality is achieved. Those layers reveal an exceptional optical quality as determined by a reduction of the low-temperature photoluminescence (PL) linewidth from 5 meV to 0.1 meV and a reduced X-ray diffraction (XRD) rocking curve width from 400 to 20 arcsec. As a consequence of the narrow PL linewidths, new features as, e. g. a fivefold fine structure of the donor-bound exciton line at 3.471 eV was detected. Additionally, all three free excitons as well as their excited states are visible in PL at 2 K.

Dry etching techniques for surface preparation allow morphologies of the layers suitable for device applications. We report on InGaN/GaN multi-quantum-well (MQW)_ structures as well as GaN pn- and InGaN/GaN double heterostructure light emitting diodes (LEDs) on GaN bulk single crystal substrates. Those LEDs are twice as bright as their counterparts grown on sapphire. In addition they reveal an improved high power characteristics, which is attributed to an enhanced crystal quality and an increased p-doping.     

Investigation of characteristics of laterally overgrown GaN on striped sapphire substrates patterned by wet chemical etching

The new developed maskless lateral-epitaxial-overgrowth technique, in which the striped substrates are patterned by wet chemical etching, is systematically investigated using scanning electron microscopy, X-ray diffraction, and atomic force microscopy (AFM). Wing tilt is measured for the GaN films on patterned substrates with a range of “fill factor” (ratio of groove width to stripe period) and for the GaN in different growth time. It is found that changes in these parameters have a significant effect on the extent and distribution of wing tilt in the laterally overgrown regions relative to the GaN directly on the sapphire substrate. Increasing the thickness of GaN films is benefit to reduce wing tilt. The tilt is avoided in the GaN films with 4.5 μm thickness and fill factor for 0.46. The full-width at half-maximum of X-ray rocking curves of the asymmetric diffraction peaks and the image of AFM both show that the threading dislocations in the developed lateral epitaxial overgrowth of GaN films are reduced sharply. The GaN template could be used as an excellent substrate to fabricate high-performance optoelectronic devices.     

Effect of growth parameters on the properties of GaN : Zn epilayers

The vapor-phase HCl/Ga/NH₃ method for deposition of GaN : Zn epilayers on sapphire substrates has been investigated to determine the dependence of epilayer resistivity, cathodoluminescence, and surface quality on the growth parameters. Both nucleation and control of the epilayer properties have been significantly improved by introducing HCl directly into the deposition zone in addition to the HCl that passes over the Ga source to produce GaCl. The effect of annealing on the stability of undoped layers and on the cathololuminescence of Zn-doped layers has also been investigated. Electroluminescent devices with reproducible properties have been obtained by growing structures consisting of an undoped n⁺ layer, a Zn-doped n-type layer, and a very thin, Zn-doped, high-resistivity layer whose growth parameters determine the emission wavelength and electroluminescence efficiency.     

Structural contribution to the roughness of supersmooth crystal surface

Technological advances in processing crystals (Si, sapphire α-Al₂O₃, SiC, GaN, LiNbO₃, SrTiO₃, etc.) of substrate materials and X-ray optics elements make it possible to obtain supersmooth surfaces with a periodicity characteristic of the crystal structure. These periodic structures are formed by atomically smooth terraces and steps of nano- and subnanometer sizes, respectively. A model surface with such nanostructures is proposed, and the relations between its roughness parameters and the height of atomic steps are determined. The roughness parameters calculated from these relations almost coincide with the experimental atomic force microscopy (AFM) data obtained from 1 × 1 and 10 × 10 μm areas on the surface of sapphire plates with steps. The minimum roughness parameters for vicinal crystal surfaces, which are due to the structural contribution, are calculated based on the approach proposed. A comparative analysis of the relief and roughness parameters of sapphire plate surfaces with different degrees of polishing is performed. A size effect is established: the relief height distribution changes from stochastic to regular with a decrease in the surface roughness.     

MOVPE growth of InN buffer layers on sapphire

We report on the MOCVD growth of InN buffer layers on sapphire substrate for InN growth. The approach used assumes that an optimized InN buffer layer has to exhibit at least the same crystalline quality and sapphire surface coverage than the GaN buffer layers allowing to grow high crystalline quality GaN on sapphire. The buffer layers were characterized by AFM and GID measurements. Sapphire nitridation was investigated: it has a strong influence on in-plane crystalline quality. Two kinds of buffer layers were optimized according to the GaN buffer layer specifications: one of them only presented In droplets at its surface. It was shown that the small amount of In droplets increases the adatoms mobility of the main layer overgrown, leading to a 25% decrease of its in-plane mosaicity, compared to InN films directly grown on sapphire. To achieve a same improvement on InN buffer layer free of In droplets, the InN main layer growth temperature had to be increased from 550 °C. to 600 °C.     

6H-SiC衬底片的表面处理

相比于蓝宝石,6H-SiC是制作GaN高功率器件更有前途的衬底.本文研究了表面处理如研磨、化学机械抛光对6H-SiC衬底表面特性的影响.用显微镜、原子力显微镜、拉曼光谱、卢瑟福背散射谱表征了衬底表面.结果表明经过两步化学机械抛光后提高了表面质量.经第二步化学机械抛光后的衬底具有优异的表面形貌、高透射率和极小的损伤层,其表面粗糙度RMS是0.12nm.在该衬底上用MOCVD方法长出了高质量的GaN外延膜.     

Study of GaN and InGaN films grown by metalorganic chemical vapour deposition

In this paper GaN and InGaN films are examined, which are grown on basal plane (0001) sapphire substrates. Growth is performed in a novel type of vertical rotating disk reactor. The effects of several growth parameters on the film quality are discussed. The GaN and InGaN layers were evaluated by surface morphology studies, DC X-ray diffraction, electrical and optical characterisation.     

Thick GaN layers on sapphire with various buffer layers

Thick GaN layers deposited in HVPE system on composite substrates made on sapphire substrates in Metalorganic Vapour Phase Epitaxy (MOVPE) system have been investigated. The following substrates were used: (00.1) sapphire substrates with AlN, AlN/GaN and GaN thin layers. The crystallographic structure and the quality of the epitaxial thick GaN layers were determined. Comparison of the three types of thick layers was performed. Significant differences were observed. It was found that thick GaN deposited on the simplest MOVPE‐GaN/sapphire composite substrate has comparable structure's properties as the other, more complicated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of the structural quality of heteroepitaxial silicon-on-sapphire structures by high-resolution X-ray diffraction,X-ray reflectivity,and electron microscopy

Heteroepitaxial silicon-on-sapphire (SOS) structures have been studied by high-resolution X-ray diffraction, X-ray reflectivity, electron microscopy, and electron diffraction. These methods yielded a large amount of complementary data on the defect structure of both the sapphire substrate and the silicon film, including integral and local (at the atomic-resolution level) information about the substrate, film, and sapphire-silicon interface.